As is known, helicopters are normally equipped with a number of drives for transmitting motion from one or more turbines to the main and/or tail rotor, and/or from the turbine to a number of accessory devices, e.g. for powering on-board instruments.
The drives normally comprise a number of gears; and a casing housing the gears and defining a compartment containing air and a lubricating oil bath, which is retained inside the drive by a number of seals fixed to the casing and operating in contact with air.
When the drive is running, the temperature and therefore pressure of the air inside the casing increases; and the increase in air pressure overloads the seals, thus resulting in oil leakage and impaired lubrication.
To reduce the pressure inside the casing in such conditions, relief devices are known to be used comprising a filter permeable to air but not to oil.
Such devices are located along a peripheral edge of the casing, and allow air to escape from the casing to restore the pressure inside the casing to practically atmospheric pressure.
Known relief devices are inefficient alongside high local pressures and particular attitudes of the helicopter, thus resulting in oil leakage and impaired lubrication of the drive.